1. Field of the Invention
The present invention relates to a semiconductor device having a multilayer interconnection structure and a method of manufacturing the same, and, more particularly, to a technique for stabilizing the contact resistance in a contact hole of a lower layer of aluminum interconnection and an upper layer of aluminum interconnection and for enhancing the reliability in the contact hole. Description of the Background Art
Various types of interconnection is normally necessary in a semiconductor device for electrically connecting elements with each other and connecting the elements with external circuits after forming the elements on a semiconductor substrate.
Conventionally, a polycrystalline silicon film, a refractory metal film, a refractory metal silicide film, an aluminum film, or an aluminum alloy film has been used as interconnection.
It is necessary to lower the wiring resistance in recent high-speed and highly-integrated devices, and a multilayer interconnection structure formed of aluminum films or aluminum alloy films having small resistivity is necessary.
FIG. 1 is a cross sectional view illustrating a structure of a DRAM (Dynamic Random Access Memory) device as an example of such a conventional semiconductor device having an aluminum multilayer interconnection structure.
Referring to FIG. 1, a conventional semiconductor device includes a DRAM element (stacked cell) 2 formed on the surface of a silicon semiconductor substrate 1 and an insulating film 3 deposited on DRAM element 2. First aluminum interconnection 4 is formed on insulating film 3, and an interlayer insulating film 5 is deposited on first aluminum interconnection 4. Second aluminum interconnection 7 is formed on interlayer insulating film 5, and a contact hole 6 is provided in interlayer insulating film 5 for connecting first aluminum interconnection 4 and second aluminum interconnection 7. A protective insulating film 8 is deposited on the semiconductor element and interconnection for protecting them from moisture and the like invading from the outside.
In a conventional semiconductor device having an aluminum multilayer interconnection structure as illustrated in FIG. 1, the stability of contact hole 6 which is a connection part of first layer of aluminum interconnection 4 and second layer of aluminum interconnection 7 is an important technical point which influences the yield or the reliability level of the device.
General description will be given of a manufacturing flow of the conventional semiconductor device illustrated in FIG. 1, mainly of the part of the step of forming contact hole 6, with reference to FIGS. 2A to 2G.
Although a combination of polycrystalline silicon interconnection, refractory metal interconnection, refractory metal silicide interconnection, and aluminum interconnection is generally used as a multilayer interconnection structure as described above, a case of an aluminum two-layer interconnection structure in which first layer of interconnection 4 and second layer of interconnection 7 are both of aluminum interconnection will be described for simplicity of description.
First, a DRAM element (stacked cell) 2 including an element isolating oxide film 301, a transfer gate electrode 302, an impurity diffused layer 303, a word line 304, a storage node 305, a capacitor insulating film 306, and a cell plate 307 is formed on the surface of a silicon semiconductor substrate 1 (FIG. 2A).
Then, a first insulating film 3 is deposited over the whole surface of silicon semiconductor substrate 1 having DRAM element (stacked cell) 2 formed thereon, and then a contact hole 308 is opened in a desired part using a photolithography process or an etching process. Then, a first layer of aluminum interconnection 4 is formed as described in the following.
In recent submicron devices, interconnection having a structure of a combination of a barrier metal film 310 formed of titanium nitride (TiN), titanium tungsten (TiW) or the like as first layer of aluminum interconnection 4 and an aluminum alloy film 311 formed of Al-Si, Al-Si-Cu or the like is used. This is because it prevents junction leakage caused by abnormal reaction (alloy spike) of first layer of aluminum interconnection 4 and the impurity diffused layer of silicon semiconductor substrate 1 in contact hole 308, and it also prevents contact failure generated by silicon in first layer of aluminum interconnection 4 being deposited in contact hole 308 by solid phase epitaxial growth, and it also enhances resistance to "stress migration" in which interconnection is disconnected by the membrane stress of interlayer insulating film 5 or protective insulating film 8 formed above first layer of aluminum interconnection 4. The films are normally deposited by a sputtering process. Thus deposited films are patterned to be first layer of aluminum interconnection 4 using a photolithography process or an etching process. "Aluminum interconnection" here also indicates one having such a multilayer structure (FIG. 2B).
Then, an interlayer insulating film 5 is deposited over the whole surface of first layer of aluminum interconnection 4. An insulating film formed of a combination of a silicon oxide film 321 deposited by a chemical vapor deposition process (CVD}, an inorganic application insulating film 322, and a silicon oxide film 323 deposited by a CVD process is used, for example, as interlayer insulating film 5 (FIG. 2C).
Silicon oxide film 321 is generally deposited by a CVD process utilizing heat or plasma at a deposition temperature in the range of 300.degree. C.-450.degree. C. using silane (SiH.sub.4) gas, oxygen (O.sub.2) gas,.or nitrous oxide (N.sub.2 O) gas. Recently, an organic silane contained material such as TEOS (Tetra-Ethyl-Ortho-Silicate) characterized by satisfactory step coverage is also used.
A material including silanol (Si(OH).sub.4) or the like as a main component is generally used as the material of inorganic application insulating film 322 used for flattening. It is applied in a rotative manner, then baked at a temperature in the range of 400.degree. C.-450.degree. C., and made into a silicon oxide film to flatten the surface of silicon oxide film 321 formed by a CVD process.
However, inorganic application insulating film 322 has high hydroscopicity and has harmful effects such as outgassing when it is exposed in a contact hole 6 which will be described later or its sidewall, so that it is etched back by a dry etching process using a fluorine contained gas or Ar gas so as not to be exposed in contact hole 6 or its sidewall.
A silicon oxide film 323 is deposited thereon by the same method as the one used for forming silicon oxide film 321.
A part of interlayer insulating film 5 where electrical connection with first layer of aluminum interconnection 4 is to be made is removed by a photolithography process and an etching process to open a contact hole 6 (FIG. 2D).
Specifically, the region except for the part 6 where a contact hole is to be opened is covered with a photoresist 324 by a photolithography process, and then interlayer insulating film 5 is selectively removed by a taper etching process in which wet etching using a hydrofluoric acid contained solution and a reactive ion etching process using CHF.sub.3, O.sub.2 and the like as a main component gas are combined, for example, to open a contact hole 6.
Photoresist 324, reaction products generated during etching and the like are removed after etching by using oxygen (O.sub.2) plasma or wet chemical processing.
The outermost surface of first layer of aluminum interconnection 4 in contact hole 6 is exposed to plasma of a fluorine contained gas such as CHF.sub.3 or oxygen gas during the step of forming contact hole 6, so that an alterated layer of aluminum (a layer formed of fluoride or oxide and having insulating properties) 201 having a thickness of approximately 100.ANG. is formed on the outermost surface of first layer of aluminum interconnection 4 in contact hole 6. In order to remove it and obtain stable contact resistance, sputter etching using Ar ions 202 is carried out first, before depositing a second layer of aluminum interconnection 7 which will be described later (FIG. 2E).
Then, a second layer of aluminum interconnection 7 is formed successively in a vacuum. An aluminum alloy film such as Al-Si, Al-Si-Cu, Al-Cu or the like is used as the material of second layer of aluminum interconnection 7. The films are deposited by a sputtering process and patterned to be interconnected by a photolithography process or an etching process as in the case of first layer of aluminum interconnection 4 (FIG. 2F).
After forming second layer of aluminum interconnection 7, heat treatment is carried out at a temperature in the range of about 400.degree. C. to about 450.degree. C. for making satisfactory electrical connection between first layer of aluminum interconnection 4 and second layer of aluminum interconnection 7 in contact hole 6.
Finally, a protective insulating film 8 formed of a silicon oxide film, a silicon nitride film or the like is deposited on second layer of aluminum interconnection 7 by a CVD process for protecting the semiconductor element and interconnection from moisture or the like invading from the outside (FIG. 2G).
Since the conventional aluminum multilayer interconnection structure is formed as described above, there is a problem of deterioration in the stability and reliability of the electrical connection between first layer of aluminum interconnection 4 and second layer of aluminum interconnection 7 in contact hole 6 of a submicron level because of reduction in its diameter according to miniaturization of the interconnection.
As described above, conventionally, sputter etching using Ar ions is carried out before depositing second layer of aluminum interconnection 7. It is carried out for removing alterated layer of aluminum (fluoride or oxide) 201 formed on the surface of first layer of aluminum interconnection 4 in contact hole 6 by using Ar ions 202, as illustrated in FIG. 2E.
In the case of the conventional device structure in which the aspect ratio B/A (A expresses the diameter of contact hole 6, and B expresses the thickness of interlayer insulating film 5) of contact hole 6 is relatively small and is 1 .mu.m or less, particles 203 of fluoride or oxide of aluminum sputtered by Ar ions 202 are drifted sufficiently outside contact hole 6 as illustrated in FIG. 3A, so that it is possible to remove alterated layer 201 of aluminum and to clean the surface.
However, in the case of contact hole 6 of a submicron level in which the aspect ratio exceeds 1, a part of particles 203 of fluoride or oxide of aluminum sputtered by Ar ions 202 are obstructed by the sidewall of contact hole 6 and cannot be drifted outside contact hole 6, and it re-attaches inside contact hole 6 to leave remaining particles 204 of fluoride or oxide as illustrated in FIG. 3B.
Therefore, even in a case where second layer of aluminum interconnection 7 is deposited successively in a vacuum, there are remaining particles 204 at the interface 205 between first layer of aluminum interconnection 4 and second layer of aluminum interconnection 7 in contact hole 6 where electrical connection is to be made as illustrated in FIG. 3C. Accordingly, mixing of first layer of aluminum interconnection 4 and second layer of aluminum interconnection 7 at interface 205 is not carried out sufficiently by heat treatment at a temperature in the range of about 400.degree. C. to about 450.degree. C. after formation of second layer of aluminum interconnection 7 as described in the above manufacturing flow.
As a result, increase in the contact resistance (hereinafter referred to as the contact hole resistance) or open failure of contact hole 6 is caused.
Even in a case where the initial value of the contact hole resistance is made normal by the above-described heat treatment at a temperature in the range of 400.degree. C.-450.degree. C., mixing at interface 205 is not carried out sufficiently, so that there is a problem of deterioration in the reliability of contact hole 6 such as resistance to electromigration or resistance to stress migration.
Another problem caused by increase in the aspect ratio of contact hole 6 is conspicuous deterioration in the coverage ratio of second layer of aluminum interconnection 7 formed by a sputtering process in contact hole 6. When coverage of aluminum in contact hole 6 is not good, the reliability in the part of contact hole 6 such as resistance to electromigration is deteriorated, and also the contact hole resistance is increased.
These problems will be more serious in the case of contact holes in submicron devices and half-micron devices in the future in which the aspect ratio becoms larger and larger.